Fannie Mae has been buying a significant volume of subprime mortgages with high loan-to-value ratios that carry no private mortgage insurance, but company officials won’t disclose how the transactions comply with the GSE’s charter restrictions. The GSE charter specifically requires that Fannie and Freddie have some form of credit enhancement on any loans they buy with LTV ratios over 80 percent. The most commonly used option is private mortgage insurance, usually written for each